The invention relates to a printing device according to the preamble of claim 1.
A printing device of the type mentioned is disclosed by DE 23 09 684 A1. This describes an electrographic printing device having an electrode comb which extends over the width of the line of print, said device comprising a lower part and an upper part mounted on the latter such that it can pivot about a pivot axis. The electrode comb and a holder for a recording medium supply roll are arranged in the lower part, and a conveying device, designed as a printing roller, for a recording medium is arranged in the upper part. The electrode comb is associated spatially with the printing roller, the individual electrodes being sprung against the printing roller. The printing roller therefore forms a print backing support and, in interaction with the electrodes, a recording medium conveying device. The upper part has two side walls which laterally enclose a recording medium supply roll put into the holder.
The known arrangement has the advantage that a recording medium supply roll can be put in particularly simply. The arrangement may also be highly suitable for a printing device which, as a print head, has an electrode comb which touches the recording medium only at a line. However, if a print head is to be used which opposes the recording medium flatly and at a distance, then this arrangement cannot be used.
WO 91 13765 A1 describes a printer which likewise comprises a lower part and an upper part that is mounted on the latter such that it can pivot about a pivot axis. A print head is arranged in the lower part, and a printing bar and a holder for a recording medium supply roll are arranged in the upper part. Provided as a conveying device for a recording medium are two pairs of conveyor rollers of which one, as viewed in the conveying direction of the recording medium, is located upstream of the printing bar and is driven at the same time as a second pair of conveying rollers, which are arranged downstream of the printing bar. It is seen as a drawback that, because of the arrangement of the recording medium supply roll in the upper part, the recording medium has to be led through a slot and around the upper conveying roller located upstream of the printing bar and also around the printing bar itself. Replacing a recording medium supply roll is therefore difficult and time-consuming.
It is therefore an object of the invention to develop a printing device of the type mentioned to the effect that a print head can be used which opposes the recording medium at a distance and whose printing elements extend at least over the width of a line of print.
For a printing device according to the preamble of claim 1, the object is achieved by the defining features of claim 1.
According to the invention, the print backing support is planar in the printing area of a serial-character or serial-column print head that can be displaced in the direction of a line of print, the conveying device comprises a pair of conveyor rollers that are aligned parallel to the printing area and can be driven by a motor, and a tautening device for the recording medium and, as viewed in the direction of a line of print, a parking area for the print head is arranged beside the printing area.
The planar configuration of the print backing support ensures that the distance to the recording medium is the same for all printing elements. The print backing support can be designed as a simple supporting plate in the case of the print head which operates without contact but, when an impact print head is used, must be designed as a printing bar. A combination, described further below, of a pair of conveyor rollers arranged on one side of the print backing support and a tautening device arranged on the other side ensures that the recording medium is always tautly tensioned in the area of the print backing support.
The print head is preferably an ink jet print head which can be moved parallel to the print backing support in the direction of a line of print and which can print one or else a number of lines of print simultaneously.
According to a development of the invention, good tensioning of the recording medium in the area of the print backing support is achieved by a pair of conveyor rollers, as viewed in the conveying direction of the recording medium, being arranged downstream of the print backing support, and a recording medium brake formed of a material with a high coefficient of friction being arranged upstream of the print backing support. In this case, the first pair of conveyor rollers pulls the recording medium held back by the recording medium brake past the print backing support. Here, the recording medium brake can consist of an elastically compressible plastic strip with a high coefficient of friction, which is supported on the print backing support.
According to an alternative development of the invention, the tensioning of the recording medium in the area of the print backing support is achieved by a first pair of conveyor rollers, as viewed in the conveying direction of the recording medium, being arranged upstream of the print backing support, and a second pair of conveying rollers, driven at the same time as the first pair of conveyor rollers, being arranged downstream of the print backing support, the second pair of conveyor rollers being driven at a slightly higher circumferential speed than the first pair of conveyor rollers and having a coefficient of friction which is slightly lower than said first pair, so that the recording medium advance step is determined by the first pair of conveyor rollers.
According to a preferred embodiment of the subject of the invention, the pair of conveyor rollers or else the first and second pairs of conveyor rollers are formed by a driven roller which is mounted in the upper part and acts on the rear of the recording medium, and a backing roller which is mounted in the lower part, is acted on with the force of a spring in the direction of the driven roller, acts on the recording side of the recording medium and runs freely. The fact that the driven rollers act on the rear of the recording medium means that smearing of the still fresh imprint is avoided. The backing rollers are carried along by the recording medium as it is advanced, that is to say moved together with the latter without slip. This reliably avoids smearing of the imprint.
The arrangement of the pivot axis of the upper part at the same height as a center line passing through the first backing roller and the first driven roller, or underneath said line, ensures that when the upper part is pivoted into its open position, the pair of conveyor rollers and the tautening device are already open after sweeping over a minimum pivoting angle, that is to say the recording medium is not carried along with it as it is pivoted.
According to a preferred embodiment of the invention, at least one latching lever provided with a claw is pivotably mounted on the lower part, and a locking pin is arranged on that side wall of the upper part which is adjacent to the latching lever. Of course, the converse arrangement is also possible. The important factor is that the two locking elements are designed to be complementary to each other.
According to an advantageous development of the subject of the invention, a two-armed unlocking lever is pivotably mounted on the side wall adjacent to the latching lever, on the side of the upper part that is remote from the pivot axis, one arm of said unlocking lever being intended to engage on the at least one latching lever with the effect of unlocking the complementary locking elements.
It is particularly advantageous if the at least one latching lever can be moved into the unlocking position only when the print head is in the parking area or an acceleration area that is located in front of the printing area, that is to say outside the printing area. As a result, during the replacement of the recording medium supply roll, the sensitive mouthpiece of the print head is protected against unauthorized contact, and the hands of the operating staff are protected against soiling.
In order to make manual displacement of the print head impossible when the printer is switched off, the print head is additionally blocked in the parking area or the acceleration area if the upper part is located outside the printing position.
In order to reduce still further the risk of smearing the still fresh ink, according to a preferred development of the invention, the backing running roller that is located downstream of the print head in the conveying direction, that is to say at least the backing roller of the pair of conveyor rollers or of the second pair of conveyor rollers, has a surface structure which cannot be wetted by printing ink and/or consists of a material which cannot be wetted by printing ink.